1. Field of the Invention
The present invention relates to an engine control apparatus and an engine operating method, and in particular, to a technology for purifying nitrogen oxides discharged from an automobile engine, using ammonia as a reducing agent.
2. Description of the Related Art
As a device for purifying air pollutants discharged from an engine, in particular, nitrogen oxides (to be referred to as NOx hereunder) in exhaust gas, using a secondary treatment, the following SCR (Selective Catalytic Reduction) device is known. This SCR device is installed in an exhaust passage of an engine, and is provided with an injection nozzle which injects an aqueous solution of ammonia or a precursor thereof. Ammonia (or ammonia obtained from the precursor) injected from the injection nozzle functions as a reducing agent, and reacts with NOx in the exhaust gas on a catalyst to reductively purify the NOx. As an SCR device that addresses ease of onboard storage of ammonia, one as described in the following is known. This SCR device is provided with a tank which stores urea as a precursor of ammonia in a state of aqueous solution, and in an actual operation, the urea water supplied from the tank is injected into the exhaust passage, so as to generate ammonia from hydrolysis of the urea using exhaust heat (Japanese Unexamined Patent Publication No. 2000-027627).
However, such an SCR device as described in the above has the following problems. As a setting related to an engine operation, there is a case where a setting for particularly reducing a particulate discharge amount is adopted. In such a setting, generally a NOx discharge amount is increased. If the SCR device is normally operated, discharged NOx can be purified by the reduction reaction with ammonia. Under such a setting in which a certain level of NOx discharge is permitted, an assumption is made on a case where an abnormality occurs in the SCR device so that a urea water injection quantity is changed or an ammonia content amount in the urea water (that is, the urea concentration) is changed. In such a case, since an ammonia addition amount to the exhaust gas is changed, a ratio between NOx and ammonia is deviated from a proper value, so that the reduction reaction is not progressed satisfactorily and the NOx removal rate does not fulfill the requirement. In particular, in the case where the ammonia addition amount is decreased, NOx is discharged into the atmosphere without being purified. Further, in the case where water or different type of aqueous solution than urea water is stored in the tank, or in the case where the tank is empty, the same result as the above is led. When such an abnormality occurs in the SCR device, it is necessary to repair the SCR device promptly, in order to suppress the NOx discharge. However, the abnormality occurred in the SCR device does not appear in the behavior of the automobile, and accordingly, a driver is unable to take notice of such an abnormality. Further, even if measures for operating a warning light or an alert are taken, it may be considered that the driver fails to perform the prompt repair.